The present invention is directed to a process and device to control the humidity of a web material, such as a paper web or a cardboard web within a color printing machine, which has several printing stations with each station printing a single basic color that is superimposed on the preceding printed color.
Such a printing machine normally comprises a lengthwise and/or sidewise registry control device for printing. The device is usually based on detection by one or several photocells, or other detectors, of the position of marks printed side-by-side and/or consecutively in the margin for each station. A calculator receives all the signals from the detectors and permanently determines the lengthwise and sidewise misregistration and applies correction signals for displacing the compensation rollers and the printing cylinders in order to rectify any misregistration.
Owing to such a control device for the registration of one printing with the others, it is possible with a precision, which is less than a tenth of a millimeter, to superimpose the printing image of one color on the other image at the place where the marks of the reference are located. However, a sidewise residual displacement of printing images outside the area where the marks are printed frequently occurs and such a displacement could be up to several tenths of a millimeter and make the final printed image unacceptable outside the areas adjoining the marks. This phenomenon is more especially visible in webs as large as three meters or thereabout, and when the printing is a fine printing, such as a negative writing because a displacement of a tenth of a millimeter is sufficient to make the printing unreadable in the area removed sidewise from the registration marks.
Afterwards, as will be understood, this sidewise residual displacement is due to, for the most part, variations of the dimensions of the web at one printing station with respect to another printing station, due to variations in the humidity of the sheet or web. As a matter of fact, in photogravure printing, for example, the ink which was made very liquid by solvents is collected outside of a pan by cells of a printing cylinder and is put by capillary action onto the paper web. This ink has to be dried before printing the next color, in order to avoid any mixing or running therebetween. This drying is realized in a progressively volatilization of the solvents between two printing groups or stations. If necessary, this volatilization is accelerated in a dryer wherein warm air or hot air is blown onto the web.
Owing to this inking and drying, the humidity of the web leaving a station depends on many factors, such as the quality of the paper, the quality of the ink, the width of the printing, the humidity of the paper before printing, the humidity and the temperature of the air in the dryer, the duration of drying and the travelling or processing speed of the web in the machine. Therefore, it was established that a variation of 1% of humidity of a paper web does change the transverse dimensions of the web by 0.1%, which means that for a web 3 meters wide, a variation of 3 millimeters will occur.
Thus, after making sure that the registry control device in the center of the image works properly, one of the machine operator's tasks consists of checking the adjustment of the printing of the webs or boards as well. If the operator detects a sidewise or transverse residual error, the operator will then intervene manually with the dryers to adjust at best the humidity of the web between the stations. This manual adjustment is, however, tedious, and only qualified persons are able to accomplish the proper adjustments.
U.S. Pat. No. 4,798,136, whose disclosure is incorporated herein by reference thereto, describes a process and device for automatic register control of printing images, excluding the printing mark in the edge and based on reading the physical width of the web at the entry of each station. The width of the web is measured by two side sensors with each sensor having a rod which at one end is carrying a wheel in contact with the web section, and the other extremity or end of the rod is carrying a magnet shifting in from of two electromagnetic windings to determine the position of the end. When the calculator detects a variation of the width of the web from one station to another, the calculator only changes the efficiency of the preceding dryer, thus, changing the temperature of the drying rollers which are in contact with the web and heated by an electrical resistance or by hot water or by vapor or the calculator modifies the airflow or the electrical heating of the air being blown into the drying station.
It is easy to understand that, contrary to what is stated in this patent, the described system could not replace an equipment of control of the lengthwise and sidewise misregistration working with reference marks in the printing. As a matter of fact, many causes for perturbation of the register will exist, for example, inhomogeneousity of the web causing a sidewise drift, variations of the coefficient of the elasticity causing a lengthwise drift, variations of the introduction tension, inertia and frictions of the idler rollers, which need a position rectification of the printing elements with the position of one element being changed with regard to the other elements. Thus, the system is not useful at all.
Moreover, regarding the correction of the variations of the printing width, in practice, the efficiency of the device is distinctly insufficient to maintain registration of the printing images, especially fine printing images, in the edge of a wide web, which is travelling at high speed. Already, when starting, it is rare that the width of the web is rigorously constant for the whole roll. The initial irregularities are inducing non-adequate corrections by this system of the patent.